The present invention relates to a process for treating a polluted aqueous liquid having a COD value caused by organic compounds present therein and a BOD/COD ratio smaller than 0.2, wherein the polluted aqueous liquid is percolated through a packed filter bed of a carrier material which is colonised with aerobic bacteria, enabling to degrade at least part of said organic compounds under aerobic conditions, and which forms an adsorbent for at least part of said organic compounds, the percolate which has passed through the filter bed is collected and a portion of the collected percolate is recirculated to the filter bed whilst a further portion of the collected percolate is removed as treated effluent showing a reduced COD value.
A BOD/COD (BOD=Biological Oxygen Demand; COD=Chemical Oxygen Demand) ratio smaller than 0.2 means that a considerable portion of the COD of the polluted aqueous liquid is recalcitrant COD or in other words that a considerable fraction of the organic compounds present in the polluted liquid is non-biodegradable. The term xe2x80x9cnon-biodegradablexe2x80x9d is used in practice and in the present specification to indicate organic compounds which are not decomposed or degraded by the micro-organisms in common waste water treatment plants but which can only be decomposed by xe2x80x9cspecialist micro-organismsxe2x80x9d and this only at a relatively low rate. The process according to the invention is in particular directed to the further purification of domestic wastewater which has already been treated biologically, i.e. to the further purification of secondary activated sludge sewage effluent which still comprises typically as major organic constituents protein, carbohydrate, humin, tanin, lignin as well as small amounts of surfactant. The process according to the invention is for example also further directed to the treatment of concentrates of reverse osmosis produced for example during the production of drinking water and containing, in addition to mineral salts, recalcitrant COD which may be caused for example by the presence of herbicides and pesticides. Of course other polluted water streams containing recalcitrant COD, in particular secondary activated sludge effluent from waste water treatment plants of the chemical or pharmaceutical industry, can also be treated.
In practice, the further treatment of such polluted water streams has become more and more important. The shortage and pollution of water resources, coupled to the continuously growing demand for clean water, has required indeed alternative ways of water conservation. By the further treatment of the effluent of waste water treatment plants to remove also non-biodegradable organic compounds therefrom, this effluent could for example be used in process water and ground water recharge applications.
WO99/37586 discloses a process for the further purification of wastewater which has already been pretreated physico-chemically and biologically. In the examples given, the pretreated waste water has a COD value of about 500 mg/l and a BOD value smaller than 15 mg/l indicating the presence of a large amount of recalcitrant COD. In this known process, the contaminated water which contains a considerable amount of non-biodegradable organic compounds is mixed with process effluent and is percolated through a filter bed of activated carbon contained in a closed reactor system. The activated carbon acts first of all as an adsorbent and as a catalyst for the biochemical oxidation of the organic compounds present in the contaminated water. An in-situ regeneration of the activated carbon is provided by colonising it with bacteria. These bacteria provide for an aerobic biological decomposition of the organic compounds. In order to provide the necessary aerobic conditions for these aerobic decomposition processes, oxygen from air or from another oxygen containing gas is dissolved in the mixture of contaminated water and process effluent which is fed into the reactor. As a result of the biological decomposition processes, the life span of the activated carbon is increased or in other words the necessary replacements of the activated carbon filter can be delayed.
In order to avoid a too high accumulation of the organic compounds in the filter bed, WO99/37586 discloses to increase the amount of oxygen dissolved in the mixture of contaminated water and effluent by pressurising this mixture to a pressure of 1 to 10 bars, preferably 1 to 3 bars above atmospheric pressure before saturating it with air. In this way an oxygen content of 10 to 100 mg O2/l, more particularly an oxygen content of 10 to 40 mg O2/l is achieved. The filter bed is further contained in a pressure vessel wherein the pressurised mixture is kept at a substantially constant pressure so that the increased oxygen concentration is maintained in the mixture of contaminated water and effluent. Due to the increased amount of oxygen in this mixture, the efficiency of the aerobic biological decomposition processes occurring in the filter bed is enhanced and accumulation of organic compounds can thus be slowed down.
A drawback of the process disclosed in WO99/37586 is that not only sufficiently powerful pumps are required to pressurise the mixture of contaminated water and effluent and the gas to be introduced in this mixture, but the filter or reactor vessel and the attendant conduits have moreover to be constructed to withstand the applied pressures. A further drawback of this known process is that a quite expensive saturation unit is required for intimately dispersing oxygen into the pressurised mixture of contaminated water and process effluent to dissolve oxygen into this mixture until it is saturated.
An object of the present invention is therefore to provide an alternative way for increasing the efficiency of the biological decomposition processes in the filter bed which does not require a saturation unit nor a pressurisation of the system.
To this end, the process according to the invention is characterised in that the filter bed is kept at the most partially submerged in the liquid percolating therethrough.
In the non-submerged part of the filter bed, the liquid fed to the filter bed flows between the particles of the carrier material leaving open pores filled with air. This air provides aerobic conditions that enable the bacteria colonising the particles of the carrier material in this part of the filter bed to perform the required aerobic decomposition processes. An advantage of at least partially not submerging the filter bed is that a higher activity of the bacteria is obtained without having to increase the amount of dissolved oxygen by pressurising to system. Indeed, it has been found that in the non-submerged part of the filter bed the bacteria grow in the form of a thin biofilm which is fully aerated. Compared to a thick biofilm, the efficiency of a thin fully aerated biofilm for performing the required aerobic decomposition processes is considerably larger.
This increased efficiency can be explained by the fact that there is only a relatively small concentration gradient of nutrients, oxygen etc. between the inner and the outer side of the film. This means that there does not have to be a large excess of oxygen in the liquid to enable an optimum metabolisation of the bacteria and that the oxygen in the air present in the pores of the non-submerged part of the filter bed can efficiently be taken up by the bacteria. A thin biofilm moreover does not substantially hamper the adsorption of the organic compounds into the pores of the carrier particles since only a small concentration gradient is required over this biofilm. Vice versa, also the supply of organic compounds adsorbed in the pores of the carrier to the bacteria is substantially not hampered by the biofilm. The bacteria in the biofilm have therefore not only constantly a sufficient supply of oxygen but dispose moreover over a source of organic compounds, namely the organic compounds adsorbed in the carrier material, containing an increased concentration of organic compounds.
In an advantageous embodiment of the process according to the invention, said further portion or in other words the treated effluent is removed from the collected percolate, which comprises a concentration of bacteria, by means of a separation device withholding the bacteria in the collected percolate to such an extend that the further portion which is removed from the collected percolate comprises a concentration of bacteria which is smaller than 10%, preferably smaller than 1%, of the bacterial concentration in the recycled percolate. In other words, the concentration of bacteria in the collected percolate is 1 log larger than, preferably 2 log larger than the bacterial concentration in the effluent. Preferably, the separation device comprises a membrane filter, in particular a micro-filtration or ultra-filtration membrane filter.
An advantage of arranging such a separation device downstream the filter bed is that specialist bacteria, which have developed onto the carrier material of the filter bed and which are able to decompose specific organic compounds which are normally non-biodegradable, are kept in the system and are recycled to the filter bed. Such a recycling offers the possibility to increase the concentration of specialist bacteria on the packed filter bed to a value required for achieving an optimal bacterial activity. In view of the normally low specific growth rate of such specialist bacteria compared to other bacteria (e.g. days instead of hours), this bacterial concentration is otherwise not easy to be reached. This is due to the fact that the specialist bacteria grow on organic compounds that are hard to metabolise.
In a preferred embodiment of the process according to the invention, at least said portion of the collected percolate which is recirculated to the packed filter bed is aerated with an oxygen containing gas, in particular with air, before recirculating it to the packed filter bed.
In this way, a larger concentration of oxygen is obtained in the liquid mixture percolating through the packed filter bed resulting therefore in a higher activity of the bacteria. According to the invention, the aeration can be carried out under atmospheric pressure or under a higher pressure in case a higher oxygen concentration is desired. A higher pressure may also be used to increase the flow rate of the liquid mixture through the packed filter bed.
The present invention also relates to an installation for treating a polluted aqueous liquid in accordance with the process according to the invention. This installation is characterised in that it comprises means for controlling the flow of liquid into and/or out of the filter bed to keep the filter bed at the most partially submerged in the liquid percolating therethrough.